Knocking on the Cell’s Door—the Holo-Translocon—So Proteins Can Enter

Though each cell is bounded by its membrane, the membrane is studded with protein complexes that manage both protein secretion and membrane insertion. If these complexes were better understood, scientists might be able to design new drugs that could more easily enter bacteria, for example. Also, it might even be possible to engineer novel membrane proteins with useful activities.

With hopes of facilitating such advances, scientists based at the University of Bristol and the European Molecular Biology Laboratory (EMBL) have managed to reconstruct and isolate a protein complex, a “holo-translocon,” that serves as a protein-trafficking machine. These scientists described their work in an article published February 18 in the Proceedings of the National Academy of Sciences (“Membrane protein insertion and proton-motive-force-dependent secretion through the bacterial holo-translocon SecYEG–SecDF–YajC–YidC”).

The scientists focused on a multiprotein complex found in Escherichia coli. This complex consists of seven membrane protein subunits, the scientists found, and includes components responsible for both protein secretion (SecYEG) and membrane protein insertion (YidC). “The availability of this intact assembly,” the scientists wrote, “allows us to reconstitute posttranslational protein export and cotranslational membrane protein insertion from purified components of known stoichiometry.”

Especially encouraging were results indicating that the activity of the translocating copy of SecYEG may be modulated by association with different accessory subcomplexes. “This versatility may provide a means to refine the secretion and insertion capabilities according to the substrate,” noted the authors. “A similar modularity may also be exploited for the translocation or insertion of a wide range of substrates across and into the endoplasmic reticular and mitochondrial membranes of eukaryotes.”

Commenting on the work, Professor Ian Collinson, from the School of Biochemistry at Bristol University said, “These findings are important as they address outstanding questions in one of the central pillars of biology, a process essential in every cell in every organism. Having unravelled how this vital holo-translocon works, we’re now in a position to look at its components to see if they can help in the design or screening for new antibacterial drugs.”